Factbox: List of U.S. billionaires making charity pledge

NEW YORK (Reuters) - A group of U.S. billionaires pledged on Wednesday to give away at least half of their wealth to charity as part of a campaign led by investor Warren Buffett and Microsoft founder Bill Gates.

Wednesday's commitments bring to 40 the number of billionaires giving away half their wealth. Many of those listed have already been engaged in significant philanthropic efforts. Below are details on how much they are worth, based on data from Forbes magazine, and how they made their fortune.

Warren Buffett -- famed investor who made his fortune through his Berkshire Hathaway firm, which buys companies he sees as undervalued; worth $47 billion

Eli Broad -- the real estate and construction billionaire founded home builder Kaufman & Broad, worth $5.7 billion; has already given more than $2 billion to The Broad Foundations; worth $5.9 billion.

John Doerr -- venture capitalist with Kleiner Perkins Caufield & Byers who made millions with early bets on Amazon, Netscape and Sun Microsystems and became a billionaire with Google; worth $1.7 billion.

Gerry Lenfest -- the media entrepreneur has already given away more than $800 million, or about 65 percent of his fortune, a spokeswoman said.